#include <iostream>
#include <cstdio>
#include <cmath>
using namespace std;

int main()
{
	double a, b, c, p;
	cin >> a >> b >> c;
	p = 1.0 / 2 * (a + b + c);
	double area = sqrt(p * (p - a) * (p - b) * (p - c));
	printf("%.1lf\n",area);
	return 0;
}
